1. Field of the Invention
The present invention relates to an antenna, and more particularly to a dipole antenna being an antenna of one wavelength of electricity having a radiation portion with an electric length of a half of a wavelength and two matching portions respectively with an electric length of a quarter of a wavelength for receiving or transmitting radio signals.
2. Description of the Related Art
With advances of technology nowadays, people can transmit information through a wireless transmission system without restriction. The antenna is an important element in the wireless transmission system. It generally is located in a wireless transmission device (such as a base station, wireless network card, bridge, router, or handset) to transform the voltage and current of a transmitter to radio signals, and broadcast the radio signals in the air by radiation. It also can receive and transform the radio signals to voltage and current to be processed in a receiver. In the prevailing trend that demands ‘slim’ and ‘light’ for wireless transmission devices, the conventional antenna is out of date. How to shrink the antenna and maintain the antenna function has become an important issue in research and development.
The commonly used dipole antenna mostly is formed in a cylindrical manner with a stripped coaxial cable housed and soldered in a metallic barrel. The copper conductive wire of the coaxial cable is connected to a transmission circuit of a circuit board in a wireless transmission device. The metallic mesh of the coaxial cable is connected to the barrel and the ground terminal of the circuit board. Its structure is generally like the one shown in FIG. 1. It includes a conductive wire 110, a ground layer 120 and a metallic duct 130. An insulation layer 112 covers the conductive wire 110. The ground layer 120 surrounds or covers the outer side of a lower terminal of the conductive wire 110 and is covered by another insulation layer 122. The metallic duct 130 is located on an upper terminal of the ground layer 120 and has an upper terminal connecting to the top terminal of the ground layer 120.
While the construction set forth above has substantially shrunk the length and size of the conventional antenna, its cylindrical shape is difficult to be installed on the wireless transmission device. Hence it usually needs a ground terminal on a distal terminal of the antenna body, to be installed securely on the wireless transmission device. Moreover, such an antenna often is extended outside the installed wireless transmission device. It is prone to be hit and damaged during transportation or use.
Refer to FIGS. 2 and 3 for a printed antenna disclosed in U.S. Pat. No. 5,754,145. It includes a dielectric substrate 200, a first element 210, two second elements 220 and 230 and a ground portion 240.
The first element 210 is located on one side of a dielectric substrate 200 and is an elongate and dipolar conductive strip. The first element 210 has a radiation portion 212 on one terminal and a feed portion 214 on other end. The second elements 220 and 230 are located on two sides of the axis of the first element 210 and have one terminal connecting to the ground portion 240 on a location about one quarter of a wavelength from the radiation terminal of the first element 210. The two second elements 220 and 230 are conductive strips at a length of one quarter of a wavelength. The ground portion 240 is located on another side of the dielectric substrate 200 opposite to the first element 210, and is also a conductive strip, which has one terminal connecting to the feed portion 214.
The printed antenna mentioned above is directly formed on a circuit board of a desired wireless transmission device in the fabrication process of the printed circuit board. While the finished wireless transmission device does not need to add an extra antenna, the fabrication cost is higher. There is still a need for a low cost antenna that is easy to install and provides required functions.